Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Uncollectable expense	$ 6	$ 7	$ 12	$ 14
Consumers Energy Company
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Uncollectable expense	6	$ 7	12	$ 14
Unbilled receivables	$ 257		$ 257		$ 409